There is an increased interest in power-scaling eye-safe technologies to the 100 Watt (W) class level. Fiber lasers, though relatively inexpensive and compact, are limited in energy and power scaling at narrow line widths from competing non-linear processes. Bulk solid-state solutions, which have large enough mode area to ignore non-linear effects, have focused on pumping at around 1400-1500 nm, where erbium-doped yttrium aluminum garnet (Er:YAG) absorption is high. The in-band pumping offers several advantages in that the quantum defect is low and there are few competing up-conversion processes. However, the high absorption limits power scaling due to thermal limitations and short gain lengths. The diode technology at these wavelengths is also less mature than at 9xx nm.
Currently, devices achieving approximately 100 W pulsed eye-safe wavelengths are restricted to in-band pumping of bulk slabs of Er:YAG. Limitations to the conventional bulk slab pumping approach include thermal handling, thermal lensing, short pump-signal interaction lengths, high upconversion levels owing to high inversions and high gain, and poor efficiency pump sources.
Another approach has been side pumping of Er:Yb:YAG in the 900 nm band where the Yb co-dopant is excited and energy is transferred to the Er species. However, the high absorption of the Yb causes similar problems as with in-band pumping of Er:YAG. Fiber lasers can achieve high power in continuous wave operation, but are severely limited in pulsed operation at narrow line widths. Beam combination can help the fiber laser solution at the cost of beam quality.
It is desirable to develop an optical system and a method for power scaling a laser to greater than 100 W, wherein the system and the method provide a compact design to manage thermal and non-linear considerations, while maximizing optical-to-optical and electrical-to-optical efficiencies.